Process to form a trench-free buried contact

ABSTRACT

A new method of forming an improved buried contact junction is described. A gate silicon oxide layer is provided over the surface of a semiconductor substrate. A polysilicon layer is deposited overlying the gate oxide layer. A hard mask layer is deposited overlying the polysilicon layer. The hard mask and polysilicon layers are etched away where they are not covered by a mask to form a polysilicon gate electrode and interconnection lines having a silicon nitride layer thereover wherein gaps are left between the gate electrode and interconnection lines. A layer of dielectric material is deposited over the substrate to fill the gaps. The had mask layer is removed. Thereafter, the polysilicon layer is etched away where it is not covered by a buried contact mask to form an opening to the semiconductor substrate. Ions are implanted into the semiconductor substrate within the opening to form the buried contact. A tungsten layer is selectively deposited overlying the buried contact and the polysilicon gate electrode and interconnection lines to form polycide gate electrodes and interconnection lines. The dielectric material layer is anisotropically etched to leave spacers on the sidewalls of the polycide gate electrodes and interconnection lines to complete the formation of a buried contact junction in the fabrication of an integrated circuit.

RELATED PATENT APPLICATION

U.S. patent application Ser. No. "09/035,139, now U.S. Pat. No. 5,998,269 issued on Dec. 7, 1999" (TSMC-97-305) to K. C. Huang et al.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming an improved buried contact without a trench in the fabrication of integrated circuits.

(2) Description of the Prior Art

FIGS. 1-3 illustrate the typical buried contact process of the prior art. FIG. 1 illustrates a partially completed integrated circuit device. Field oxide regions, such as 12, are formed in and on the semiconductor substrate 10. A gate oxide layer 14 is grown upon the surface of the substrate. Typically, a so-called "split poly" process is used wherein a first layer of polysilicon 16 is deposited over the gate oxide layer to protect the gate oxide from the photoresist process. A layer of photoresist is coated over the polysilicon layer 16 and patterned to form the photoresist mask 20.

The polysilicon and gate oxide layers are etched away where they are not covered by the photoresist mask to form an opening where the buried contact is to be formed. As illustrated in FIG. 2, the second layer of the split poly 22 is deposited over the first polysilicon layer and within the opening. A second photoresist mask 24 is formed over the substrate.

Referring now to FIG. 3, dopant from the polysilicon layer 22 is driven in to form the buried contact 26 and the polysilicon and gate oxide layers are etched to form gate electrode 28 and polysilicon interconnection line 30. Source/drain regions 32 are formed.

As device dimensions and cell size continue to decrease for high density and improved performance in integrated circuits, there is a growing demand for lower junction leakage and lower contact resistance. However, the contact resistance and junction leakage will increase in the conventional buried contact process if there is misalignment of the photoresist mask during polysilicon etching.

FIG. 4 illustrates the case in which the photoresist mask 24 is shifted to the left. Buried contact trench 35 is formed. This causes an increase in both contact resistance and leakage current. FIG. 5 illustrates the case in which the photoresist mask is shifted to the right. A disconnection gap 37 is left between the buried contact 26 and the source/drain region 32. This increases contact resistance by causing a high series resistance. The dotted lines in each figure illustrates the current path.

A number of patents disclose methods for improving a device in which a buried contact trench has been formed. For example, U.S. Pat. No. 5,525,552 to J. M. Huang teaches the use of a low dielectric constant spacer to provide better immunity of the buried contact trench. U.S. Pat. No. 5,607,881 also to J. M. Huang teaches linking the buried contact junction and the source junction by an extra high dosage N+ implant to overcome the disadvantages of a buried contact trench. U.S. Pat. No. 5,668,051 to Chen et al teaches a thin polysilicon layer within the buried contact trench. U.S. Pat. No. 5,652,152 to Pan et al discloses the use of a PSG spacer to solve the buried contact trench problem.

Other patents teach methods to avoid forming a buried contact trench. For example, U.S. Pat. No. 5,494,848 to H. W. Chin teaches the use of a reverse tone oversized buried contact mask to prevent formation of a buried contact trench. U.S. Pat. No. 5,654,231 to M. S. Liang et al teaches the use of sidewall spacers to prevent the formation of a buried contact trench in DRAM technology. Co-pending U.S. patent application Ser. No. 09/035,139, now U.S. Pat. No. 5,998,269 issued on Dec. 7, 1999 (TSMC-97-305) to K. C. Huang et al teach a method of forming polysilicon gate electrodes and interconnection lines before forming a buried contact and depositing a refractory material layer over the buried contact.

Still other patents teach other buried contact processes. For example, U.S. Pat. No. 5,332,913 to Shappir discloses a buried interconnect structure in which a contact is formed over a field oxide region. A poly-epi silicon layer is formed over the contact and another oxidation region is formed over the poly-epi silicon layer. U.S. Pat. No. 5,543,362 to Wu teaches a process in which a silicide layer is deposited over the buried contact region followed by a polysilicon layer and topped with a second silicide layer. U.S. Pat. No. 5,162,259 to Kolar et al teaches forming a silicide over the buried contact region and depositing polysilicon overlying the silicide.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an effective and very manufacturable method of forming buried contact junctions.

Another object of the present invention is to provide a method of forming buried contact junctions which avoids the formation of a trench caused by mask misalignment.

Another object of the invention is to provide a method of forming buried contact junctions which avoids the formation of a disconnection gap caused by mask misalignment.

Yet another object of the present invention is to provide a method of forming buried contact junctions in which only a single deposition of polysilicon is required.

A further object of the invention is to provide a method of forming buried contact junctions in which a selective tungsten process is used to form the polycide gate and interconnection lines.

A still further object is to provide a method of forming buried contact junctions which avoids the formation of a trench caused by mask misalignment and in which only a single deposition of polysilicon is required.

A still further object is to provide a method of forming buried contact junctions which avoids the formation of a trench caused by mask misalignment, in which only a single deposition of polysilicon is required, and in which a selective tungsten process is used to form the polycide gate and interconnection lines.

In accordance with the objects of this invention a new method of forming an improved buried contact junction is achieved. A gate silicon oxide layer is provided over the surface of a semiconductor substrate. A polysilicon layer is deposited overlying the gate oxide layer. A hard mask layer is deposited overlying the polysilicon layer. The hard mask and polysilicon layers are etched away where they are not covered by a mask to form a polysilicon gate electrode and interconnection lines having a hard mask layer thereover wherein gaps are left between the gate electrode and interconnection lines. A layer of dielectric material is deposited over the semiconductor substrate to fill the gaps. The hard mask layer is removed. Thereafter, the polysilicon layer is etched away where it is not covered by a buried contact mask to form an opening to the semiconductor substrate. Ions are implanted into the semiconductor substrate within the opening to form the buried contact. A tungsten layer is selectively deposited overlying the buried contact and the polysilicon gate electrode and interconnection lines to form polycide gate electrodes and interconnection lines. The dielectric material layer is removed. An oxide layer is deposited and anisotropically etched to leave spacers on the sidewalls of the polycide gate electrodes and interconnection lines to complete the formation of a buried contact junction in the fabrication of an integrated circuit.

Also, in accordance with the objects of this invention, a new integrated circuit device having a buried contact junction is described. A buried contact junction lies within a semiconductor substrate having a titanium silicide layer thereover and a tungsten contact layer overlying the titanium silicide. A polycide gate electrode lies on the surface of the semiconductor substrate having source and drain regions within the semiconductor substrate surrounding the polycide gate electrode wherein one of the source and drain regions contacts the buried contact junction. An insulating layer overlies the said polycide gate electrode, the tungsten contact layer, and the source and drain regions. A patterned conducting layer overlies the insulating layer and extends through an opening in the insulating layer to another one of the underlying source and drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this description, there is shown:

FIGS. 1 through 5 schematically illustrate in cross-sectional representation a buried contact process of the prior art.

FIGS. 6 through 13 schematically illustrate in cross-sectional representation a preferred embodiment of the present invention.

FIGS. 12A and 12B schematically illustrate in cross-sectional representation two alternatives in a preferred embodiment of the present invention.

FIG. 14 schematically illustrates in cross-sectional representation a completed static random access memory (SRAM) fabricated according to the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now more particularly to FIG. 6, there is shown a portion of a partially completed integrated circuit. There is shown a monocrystalline semiconductor substrate 10 in which there are formed field oxide regions 12. A layer of gate oxide 14 is grown over the surface of the substrate, typically to a thickness of between about 40 to 250 Angstroms.

Next, polysilicon layer 18 is deposited over the gate oxide layer 14. Usually, a split polysilicon layer is used. A thin polysilicon layer would be deposited at this time to protect the gate oxide while the opening for the buried contact region is etched. A thick layer of doped polysilicon would then be deposited within the buried contact opening and dopant would be driven in from the thick polysilicon layer to form the buried contact. However, in the process of the invention, a single polysilicon layer can be used without degrading the quality of the gate oxide. Thus, the process of the invention has a lower thermal budget. The single polysilicon layer 18 of the invention has a thickness of between about 1000 to 3000 Angstroms.

A thin layer of silicon oxide 19 is grown or deposited over the polysilicon layer 18 to a thickness of about 100 Angstroms.

Next, a layer of silicon nitride or other dielectric material 21 is deposited overlying the polysilicon layer 18 to a thickness of between about 1000 and 3000 Angstroms, as a hard mask. A layer of photoresist is coated over the silicon nitride layer and exposed, developed, and patterned to form the photoresist mask 23. This is the mask to form the gate electrode and interconnection lines.

Referring now to FIG. 7, the silicon nitride, silicon oxide, and polysilicon layers 21, 19, and 18 are etched away where they are not covered by the mask to form the polysilicon structures that will become gate electrode 40 and interconnection lines 42. Ions 33 are implanted to form lightly doped source/drain (LDD) regions 34. Alternatively, the LDD regions can be formed later in the process.

Now, the openings between the gate electrode and the interconnection lines are filled with an oxide material 44 deposited by chemical vapor deposition (CVD). The oxide may be sub-atmospheric CVD oxide, high density plasma CVD oxide, or the like. A spin-on-glass material may also be deposited to completely fill the openings and then etched back to result in filled openings and a planarized substrate, as illustrated in FIG. 8.

Referring now to FIG. 9, the silicon nitride layer 21 is etched away, for example, using H₃ PO₄ at 100 to 200° C. The buried contact photoresist mask 20 is now formed over the wafer, as shown in FIG. 10. The mask may be modified by shifting the edge slightly to the right in order to avoid the misalignment problem leading to a disconnection gap. The polysilicon 18 is etched away to open the buried contact, as illustrated in FIG. 11. Phosphorus or Arsenic ions 46 are implanted into the substrate within the opening to form implanted region 48 which will form the buried contact junction. Ions are implanted with a dosage of between about 1 E 12 to 1 E 14 atoms/cm² at an energy of between about 30 to 60 KeV.

Since the buried contact opening is made after the gate electrode and interconnection line polysilicon Patterning, there is no need to overlay the polysilicon mask. There will be no buried contact trench due to mask misalignment during polysilicon overetching. This allows the design rule of the polysilicon and buried contact masks to be tighter; hence device density can be increased. Since there will be no buried contact trench, the contact resistance and junction leakage can be decreased significantly. As mentioned above , modifying the mask may be done, if necessary, to avoid a disconnection gap.

Selective tungsten technology is used to form the polycide gate and interconnection lines. Optionally, a layer of titanium can be deposited over the surface of the substrate and annealed to form titanium silicide overlying the polysilicon layer 18 and the silicon substrate at the buried contact 48. The unreacted titanium is removed to leave titanium silicide layer 52, as illustrated in FIG. 12A, overlying the polysilicon lines and the silicon substrate over the buried contact. This titanium silicide layer reduces contact resistance.

Next, tungsten 54 is selectively grown over the polysilicon layer 18 and the silicon substrate at the buried contact junction 48, as illustrated in FIG. 12B, or over the titanium silicide layer 52, as shown in FIG. 12A,

The oxide 44 between the gate electrode and interconnection lines is removed. If the LDD regions 34 have not yet been formed, they may be implanted at this time. Another dielectric layer is deposited and anisotropically etched to leave spacers 56 on the sidewalls of the gate electrode and interconnection lines, as shown in FIG. 13. Heavily doped source and drain regions 58 are implanted into the substrate using the gate electrode as a mask.

Processing continues as is conventional in the art to complete the integrated circuit device. FIG. 14 illustrates a cross-sectional view of a completed SRAM integrated circuit device with electrical connections. An insulating layer 60 is deposited overlying the gate electrode 40 and interconnection lines 42. The insulating layer may be composed of a thin layer of undoped oxide and a thick layer borophospho-TEOS (BP-TEOS) or borophosphosilicate (BPSG). An opening is etched through the insulating layer to one of the underlying source/drain regions 58. A metal layer 62 is deposited over the surface of the substrate and within the opening and patterned. An intermetal dielectric 64 is deposited, followed by the deposition and patterning of a second metal layer 66. A passivation layer 68 completes the SRAM integrated circuit device.

The integrated circuit device having a buried contact junction according to the present invention is described with reference to FIG. 14. FIG. 14 illustrates a cross-sectional view of a completed SRAM integrated circuit device with electrical connections. Buried contact junction 48 lies within the semiconductor substrate 10. A tungsten contact layer 54 overlies the buried contact junction. A titanium silicide layer 52 lies between the buried contact junction and the tungsten contact layer 54. A polycide gate electrode 40 lies on the semiconductor substrate. Associated source and drain regions 58 lie on either side of the gate electrode 40 within the semiconductor substrate. One of the source and drain regions 58 contacts the buried contact region 48. An insulating layer 60 covers the gate electrode 40 and interconnection lines 42. A metal layer 62 lies over the surface of the insulating layer 60 and extends through an opening within the insulating layer to another one of the underlying source and drain regions 58. An intermetal dielectric layer 64 overlies the metal layer 62. A second patterned metal layer 66 overlies the intermetal dielectric layer. A passivation layer 68 completes the SRAM integrated circuit device.

The process of the invention avoids the etching of a trench into the buried contact junction because the polysilicon gate electrode and interconnection lines are formed before the buried contact. The polysilicon gate electrode and interconnection lines are formed before the buried contact opening is etched so that it is not necessary to overlay the polysilicon and buried contact masks. This results in a trench-free buried contact process with significantly decreased contact resistance and junction leakage. Since the masks do not have to be overlaid, the design rules of the masks can be tighter, thereby increasing device density. Selective tungsten deposition is used to form the contact to the buried contact and to form polycide gate electrodes and interconnection lines. The process of the invention can be used in the fabrication of any integrated circuit device having a buried contact and polycide gate electrodes and interconnection lines, such as the static random access memory (SRAM) device illustrated in FIG. 14.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A method of forming a buried contact junction in a semiconductor substrate in the fabrication of an integrated circuit with local interconnect comprising:providing a gate silicon oxide layer over the surface of said semiconductor substrate; depositing a polysilicon layer overlying said gate oxide layer; depositing an oxide layer overlying said polysilicon layer; depositing a hard mask layer overlying said oxide layer; etching away said hard mask layer, said oxide layer, and said polysilicon layer where they are not covered by a mask to form a polysilicon gate electrode and interconnection lines having said hard mask layer thereover wherein gaps are left between said gate electrode and said interconnection lines; depositing a layer of dielectric material over said semiconductor substrate to fill said gaps; removing said hard mask layer; thereafter etching away said polysilicon layer where said polysilicon layer is not covered by a buried contact mask to form an opening to said semiconductor substrate; implanting ions into said semiconductor substrate within said opening to form said buried contact junction; selectively depositing a tungsten layer overlying said buried contact junction and said polysilicon gate electrode and interconnection lines to form polycide gate electrode and interconnection lines; removing said dielectric material layer; and forming dielectric spacers on the sidewalls of said polycide gate electrode and interconnection lines to complete the formation of said buried contact junction in the fabrication of said integrated circuit with local interconnect.
 2. The method according to claim 1 wherein said polysilicon layer has a thickness of between about 1000 and 3000 Angstroms.
 3. The method according to claim 1 wherein said hard mask layer comprises silicon nitride layer and has a thickness of between about 1000 and 3000 Angstroms.
 4. The method according to claim 1 wherein said dielectric material layer comprises silicon oxide.
 5. The method according to claim 1 wherein said dielectric material layer comprises an oxide and a spin-on-glass material which is etched back to completely fill said gaps.
 6. The method according to claim 1 further comprising depositing ions into said semiconductor substrate within said gaps to form lightly doped source and drain regions before said step of depositing said layer of dielectric material.
 7. The method according to claim 1 further comprising depositing ions into said semiconductor substrate within said gaps to form lightly doped source and drain regions after said step of removing said layer of dielectric material.
 8. The method according to claim 1 further comprising forming a titanium silicide layer overlying said buried contact junction and said polysilicon gate electrode and interconnection lines before said step of selectively depositing said tungsten layer.
 9. The method according to claim 1 further comprising implanting ions into said semiconductor substrate to form heavily doped source and drain regions using said polycide gate electrode as a mask after said step of forming said dielectric spacers.
 10. The method according to claim 9 further comprising:covering said polycide gate electrode and interconnection lines with an insulating layer; opening a via to one of said heavily doped source and drain regions within said semiconductor substrate; and filling said via with a metal layer to complete electrical connections in the manufacture of said integrated circuit.
 11. A method of forming a buried contact junction in a semiconductor substrate in the fabrication of an integrated circuit with local interconnect comprising:providing a gate silicon oxide layer over the surface of said semiconductor substrate; depositing a polysilicon layer overlying said gate oxide layer; depositing a silicon nitride layer overlying said polysilicon layer; etching away said silicon nitride layer and said polysilicon layer where they are not covered by a mask to form a polysilicon gate electrode and interconnection lines having a silicon nitride layer thereover wherein gaps are left between said gate electrode and said interconnection lines; implanting first ions through said gaps into said semiconductor substrate to form lightly doped source and drain regions; depositing a layer of dielectric material over said semiconductor substrate to fill said gaps; removing said silicon nitride layer; etching away said polysilicon layer where it is not covered by a buried contact mask to form an opening to said semiconductor substrate; implanting second ions into said semiconductor substrate within said opening to form said buried contact junction; selectively depositing a tungsten layer overlying said buried contact function and said polysilicon gate electrode and interconnection lines to form polycide gate electrode and interconnection lines; removing said dielectric material layer; forming spacers on the sidewalls of said polycide gate electrode and interconnection lines; and implanting third ions into said semiconductor substrate using said polycide gate electrode as a mask to form heavily doped source and drain regions to complete the formation of said buried contact junction in the fabrication of said integrated circuit with local interconnect.
 12. The method according to claim 11 wherein said polysilicon layer has a thickness of between about 1000 and 3000 Angstroms.
 13. The method according to claim 11 wherein said silicon nitride layer has a thickness of between about 1000 and 3000 Angstroms.
 14. The method according to claim 11 wherein said dielectric material layer comprises silicon oxide.
 15. The method according to claim 11 wherein said dielectric material layer comprises an oxide and a spin-on-glass material which is etched back to completely fill said gaps.
 16. The method according to claim 11 further comprising forming a titanium silicide layer overlying said buried contact and said polysilicon gate electrode and interconnection lines before said step of selectively depositing said tungsten layer.
 17. The method according to claim 11 further comprising:covering said polycide gate electrodes and interconnection lines with an insulating layer; opening a via to one of said heavily doped source and drain regions within said semiconductor substrate; and filling said via with a metal layer to complete electrical connections in the maufacture of said intergrated circuit. 